Traditional twisted-pair telephone lines can be used to provide voice telephone service to customers. A central office can connect to a customer premises via a transmission line, such as a twisted-pair copper line. Once at the customer premises, the transmission line may connect to wiring within the customer premises. The customer wiring may split into more than one branch, and the branches may run to various different locations within the customer premises. Plain old telephone service (“POTS”) devices, such as analog telephones, can connect to the customer wiring branches. Through the customer wiring, the POTS devices can interface with the central office and ultimately communicate with other devices also connected to the network.
In addition to carrying voice calls, the transmission line can also carry data. For example, computers, fax machines or other data devices can connect to the customer wiring. Then they can communicate with the central office, and ultimately with other devices connected to the network. In this type of configuration, the data is transmitted over the bandwidth generally allocated for voice calls. The bandwidth for voice calls is generally limited, and this also limits the data transmission rate. Additionally, a standard telephone line generally cannot support simultaneous data and voice transmissions.
XDSL is a technology that overcomes some of the limitations of POTS service. The acronym XDSL generally identifies the broad class of “digital subscriber line” services. The letter “X” in the general acronym can be replaced with another letter to identify a specific type of digital subscriber line service, such as asymmetric digital subscriber line (“ADSL”) service or high-bit-rate digital subscriber line (“HDSL”).
Using XDSL, the bandwidth of the telephone line can be increased to allow faster data communication than POTS service. For example, by changing bandwidth-limiting filters on the network, the overall available bandwidth for the transmission line connected to the customer premises can be increased. A portion of the available bandwidth can be used to support POTS services, such as a voice call. Another portion of the bandwidth can be used to support high-speed data communications. By using separate bandwidths for POTS and XDSL data, the XDSL system can support simultaneous voice and data communications.
In the XDSL system, the available bandwidth can be divided into three ranges. A POTS frequency range can be used to support POTS services, and two frequency ranges can be used to support XDSL services. An upstream frequency range can be used for XDSL communication from the customer premises to the central office, and a downstream frequency range can be used for XDSL communication from the central office to the customer premises. In order to prevent one frequency range from adversely interfering with another frequency range, the frequency ranges can be separated by stopbands.
Various different problems can occur within the XDSL system that can adversely affect the overall data transmission rate. For example, a hybrid may be used to separate transmit and receive signals at a 4-wire to 2-wire interface, such as at the interface of the transmission line and the customer premises. The hybrid can send signals from the central office to an XDSL device on a receive path, and it can receive transmit signals from the XDSL device on a transmit path. The hybrid can include an impedance matched to the transmission line. If the impedance of the hybrid matches closely to the impedance of the transmission line, then the hybrid may be able to effectively cancel transmit signals from the receive path. However, if the impedance does not closely match that of the transmission line, then the hybrid may not effectively cancel transmit signals from the receive path, thereby decreasing the overall transmission rate in the receive path.
Bridge taps may adversely affect the performance within the transmission system. A bridge tap can be, for example, a non-terminated copper pair wire connected in parallel to a 2-wire line. Bridge taps can be located on the transmission line, on the wiring within the customer premises or on both locations. The bridge taps can alter the impedance of the transmission line as seen looking from the hybrid toward the central office, thereby adversely affecting the cancellation properties of the hybrid. The bridge taps can cause an impedance mismatch in the hybrid, and the impedance mismatch can cause signals in the upstream frequency range to be reflected into the downstream frequency range. The reflection can decrease the data transmission rate in the downstream direction.
Therefore, there exists a need to provide a better way to increase performance in an XDSL system.